Pressure balancing valve stuck

I have a pressure balancing valve problem: no hot water coming out of the shower. The valve is made by Tempra, it is separate from the shower valve, about 3 years old. I have never actually seen inside one of these things, so what will I find inside and how easy is it to fix? Should I get a rebuild kit before I open it up?

You may be able to remove it and then tap it on the floor to free it, but if if it breaks during the removal you will not be able to turn the water on until you get a new one. At the very least I would be sure I knew where I could get the new one before I started.